1. Field of the Invention
The invention relates to a steering transaxle having a hydraulic motor for driving steerable wheels. The invention also relates to a hydraulic driving vehicle, especially a four-wheel driving vehicle, e.g., an agricultural tractor, a riding mower and a construction machine, comprising the steering transaxle.
2. Related Art
As disclosed in the Japanese Patent No. 2594951 (document '951) and the Japanese Laid Open Gazettes Nos. 2002-172946 (document '946) and 2000-1127 (document '127), there are conventional well-known four-wheel driving vehicles, each of which has a front steering transaxle and a rear unsteering transaxle. The front steering transaxle supports steerable front wheels and incorporating a front differential gear unit mutually differentially connecting the front wheels, and the rear unsteering transaxle supports unsteerable rear wheels and incorporating a rear differential gear unit mutually differentially connecting the rear wheels, wherein the speed ratio between the steerable wheels and the unsteerable wheels is changed according to the steered angle of the steerable wheels.
The vehicle disclosed in the document '946 is provided with a large and complicated mechanical (planetary gearing) center differential gear unit interposed between the front steering transaxle and the rear unsteering transaxle. A large middle space of the vehicle is necessary for arranging the center differential gear unit, and two front and rear propeller shafts for transmitting power of the center differential gear unit to the respective front and rear transaxles, thereby causing designing limitation.
With respect to the conventional hydraulic driving vehicles disclosed in the documents '951 and '127, instead of the center differential gear unit with the front and rear propeller shafts, each of the vehicles is provided with a variable displacement hydraulic motor for driving the front differential gear unit, a fixed displacement hydraulic motor for driving the rear differential gear unit, and a common hydraulic pump for supplying both the hydraulic motors with hydraulic oil one after another, thereby increasing a free space for arranging various parts.
With respect to the disclosed conventional hydraulic driving vehicles, it is desired for smoothly driving the steerable front wheels that the displacement of the front hydraulic motor is reduced as far as possible so as to correspond to the reduced hydraulic oil after passing the rear hydraulic motor. However, such displacement of the front hydraulic motor causes increase of the rotary speed of the front hydraulic motor, thereby requiring increase of the deceleration ratio between the front hydraulic motor and the steerable front wheels so as to be balanced with the rotary speed of the unsteerable rear wheels. Upsizing of a deceleration gear train between the front hydraulic motor and the front differential gear unit for ensuring the increased deceleration ratio is not desired because it causes the front steering transaxle incorporating the deceleration gear train to expand the vehicle vertically and lengthwise so as to reduce ground clearance below.
Further, the rotary axis of the front hydraulic motor (its motor shaft) is disposed perpendicularly to that of the axles of the front wheels. If the front hydraulic motor and the front differential gear unit are disposed in a transaxle casing, the transaxle casing must be large lengthwise of the vehicle so as to ensure the length of the motor shaft.
Further, if a steering transaxle incorporating a variable displacement hydraulic motor, a deceleration gear train and a differential gear unit is to be constructed, it requires many parts and units to be assembled and properly located. Easy assemblage of the steering transaxle such as to overcome the difficulty of assembling and locating many parts and units is desired for reducing labor and time.
Further, the vehicle disclosed in the document '127 is provided with brakes in the rear transaxle so as to individually brake the respective unsteerable rear wheels. Since the vehicle has the variable displacement hydraulic motor for driving the steerable front wheels, and the fixed displacement hydraulic motor for driving the unsteerable rear wheels, the steerable front wheels are accelerated and the unsteerable rear wheels are not reduced during turning of the vehicle. In brief, the high speed of the unsteerable rear wheels that occurs during straight traveling of the vehicle is kept even during turning of the vehicle, thereby increasing a centrifugal force applied onto the turning vehicle and making the vehicle unstable. Further, in this condition during turning of the vehicle, if the vehicle brake-turns, i.e., if one of the brakes is actuated for braking, a braking shock may occur.